In CCD image sensors and CMOS image sensors, the number of photons that enter a unit pixel decreases as the pixel size becomes smaller. As a result, the sensitivity becomes lower, and the S/N ratio also becomes lower. Further, in an image sensor having a pixel array in which pixels to receive red, green, and blue light are arranged in a plane, such as the Bayer array that involves primary color filters and is widely used today, green and blue light does not pass through the color filters in the red pixels, and cannot be used in photoelectric conversion. As a result, loss is caused in terms of sensitivity. Further, false colors are also generated, as interpolation processing between pixels is performed to create color signals.
As a method for solving those problems, there are known image sensors in which three photoelectric conversion layers are stacked in the vertical direction to obtain photoelectric conversion signals of three colors with one pixel. As such a structure in which photoelectric conversion layers of three colors are stacked for one pixel, for example, there is a suggested sensor in which a photoelectric conversion unit that detects green light and generates a signal charge corresponding to the detected green light is provided above a silicon substrate, and blue light and red light is detected with two PDs stacked in the silicon substrate (Patent Document 1, for example).
There also is a suggested back-illuminated solid-state imaging device in which the circuit formation surface is formed on the opposite side from the light receiving surface in a structure in which an organic photoelectric conversion film (a photoelectric conversion unit) that photoelectrically converts light of one color is provided above a silicon substrate, and a two-layer photoelectric conversion unit that photoelectrically converts light of two colors are provided in the silicon substrate (Patent Document 2, for example). In a case where a three-layer photoelectric conversion unit is formed in a back-illuminated structure, circuits, wiring lines, and the like are not formed between an inorganic photoelectric conversion unit and an organic photoelectric conversion unit, so that the inorganic photoelectric conversion unit and the organic photoelectric conversion unit in the same pixel can be made closer to each other. Accordingly, the dependence of each color on the F value can be reduced, and variation of sensitivity among the respective colors can be reduced.
Since an organic photoelectric conversion film provided above a silicon substrate is easily damaged by moisture, oxygen, or the like during and after the manufacturing, there is a need for a sealing film that protects the organic photoelectric conversion film. In a structure in which a photoelectric conversion unit is formed by stacking a lower electrode, an organic photoelectric conversion film, and an upper electrode in this order on a silicon substrate, it is necessary to protect the upper electrode and the side walls thereof with a sealing film. In particular, at the side wall portions, the organic photoelectric conversion film is exposed to separate the upper electrode and the lower electrode from each other, and therefore, protection with a sealing film is critical.
In Patent Document 3, AlO according to an ALD method is formed as a sealing film having excellent protection characteristics on the upper electrode. Further, Patent Document 4 suggests protection of the top portion and the side walls of an organic photoelectric conversion film with a sealing film containing silicon nitride as a principal component.